This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Y-family polymerases are main players in translesion (DNA damage bypass) DNA synthesis, which are characterized by their ability to replicate transverse lesions and by their reduced fidelity in DNA replication. Due to their mutagenic potential, Y-family polymerase activities must be tightly regulated. It has been demonstrated that sliding clamp proteins (PCNA) play an important role for the regulation as a co-factor. The activities of the Y-family polymerases and their ability to bypass replication-blocking lesions are regulated by their cofactors. The interactions between PCNA and DNA polymerases play a role in coordinating different polymerases during replication. We are working on X-ray structures of complexes of Y-family polymerases in complex with DNA fragments containing different lesions and mismatches to decipher its specificity in lesion bypass. In addition, structural analyses of Y-family polymerases in complex with co-factors are undertaken to reveal the mechanism of multiple polymerase switching in replication forks. Carcinogenesis and anti-cancer drug resistance are major focuses of cancer research. Both processes are associated with a newly identified DNA polymerase family, the Y family. Benzo[a]pyrene (BP) is a ubiquitous environmental pollutant and a carcinogen. The BP-adducts in DNA cause elevated mutagenesis in cells and leads to a variety of cancers. It is the Y-family polymerases that make extra mutations when they replicate past lesions erroneously. Cisplatin is one of the most widely used chemotherapeutic drugs for human cancers. However, intrinsic or acquired resistance to cisplatin constitutes a major limitation in its application. Cisplatin exerts its cytotoxicity through the formation of covalent adducts in DNA. These adducts inhibit DNA synthesis in rapidly dividing tumor cells. Lesion bypass replication causes drug-resistance to cisplatin in cells, which are mainly performed by Y-family polymerases. We use Y-family polymerase Dpo4 as an in vitro system and to explore the structural details of how the Y-polymerase interacts with BP- &cisplatin- DNA adducts that are closely associated with cancers and cancer treatment. The structural analysis will reveal specific interactions of PCNA with translesion polymerases. Knowledge of the detailed interactions between the protein and lesion substrates will help to understand the damage-induced mutagenesis and provide a molecular basis for DNA damage tolerance that renders cisplatin resistance in cancer treatment. The results will be invaluable in rational drug design to treat cancer.